1. Field of the Invention
The present invention relates to a method of correcting a change of position of a machine tool controlled by a numerical control device, and more particularly, to a position correction method in which a change of position of a machine tool with control axes having different rigidities is corrected.
2. Description of the Related Art
In a numerical control device (CNC), a workpiece is machined to a desired contour by moving a tool along a commanded path at a commanded speed in accordance with a machining program.
To obtain a satisfactorily machined surface produced in accordance with commands given to a machine tool, using such a numerical control device, it is essential to provide a servo mechanism having a responsiveness such that it can faithfully follow rapid changes of commands and having a high stability enabling the maintaining of a stable movement without vibration.
The speed and position of a servomotor in the servo mechanism are detected by a speed detector and a position detector, and the data thus detected is fed back to a control circuit for control. The servo mechanism can be classified into three types according to the method used for the position detection; i.e., a semi-closed-loop type, a closed-loop type, and a hybrid servo type.
Among these three methods used for the servo mechanism, a method best suited to each machine tool is employed according to a required accuracy and rigidity, etc. of the machine tool.
In ordinary machine tools, however, the individual axes have different mechanical rigidities, and this difference becomes particularly large in large-sized machine tools. Therefore, when a cutting is effected by a machine tool with more than two axes, machining errors occur in the vicinity of the machining start and end points or at corners, and thus the cutting cannot be effected with a high accuracy.
FIG. 4 shows a cutting with two axes according to the prior art. In this example, it is assumed that the X-axis has a relatively weak mechanical rigidity and the Y-axis has a relatively strong mechanical rigidity, and with these control axes, when there is movement along the X-axis 0.100 mm, of the movement along Y-axis is 0.200 mm.
If the mechanical rigidities of the X- and Y-axes are the same, a linear cutting with no error is usually effected as indicated by the straight line A, but if mechanical rigidity of the X-axis is weak, a required movement along the X-axis is not achieved, as indicated by the curve B1, and even though the ratio between the distribution pulses for the X-axis and Y-axis is 1:2, an actual ratio of machine position becomes 1:4, as shown at the point C, thus causing a machining error.